1. Field of the Invention
The present invention relates to a liquid crystal display device those and more particularly to a liquid crystal display device which can control a viewing angle.
2. Discussion of the Related Art
Generally, liquid crystal display devices, especially, using a thin film transistor (TFT), are used in mobile phones, large scale televisions and the like. Of them, some display devices for a personal purpose are required to satisfy a condition that a display is viewable to only a user and not viewable to others who look at the display from the slanted viewing direction.
It is more preferable that display devices are configured to have different display modes such that a display is viewable to all observers in some cases and viewable to only a user in other cases.
FIG. 8 is a view for explaining a related art liquid crystal display device having a confidential mode.
A related art liquid crystal display (LCD) device having a confidential mode illustrated in FIG. 8 has been proposed (see Japanese Patent Laid-open Publication No. 5-72529). A backlight having a high directivity is used for illuminating an LCD panel from the back.
Means for converting a light-scattering state into a non-scattering state and vice versa, for example, a polymer dispersed LCD panel (a scattering/non-scattering switching layer), is provided between the common LCD panel and the directional backlight. When the switching layer is in a non-scattering state, because light emitted from the backlight is irradiated to only the front, the display is invisible from the slanted viewing direction.
On the other hand, when the switching layer is in a scattering state, because light emitted from the backlight spreads in different directions, all observers can see the display from the slanted viewing direction as well as the front. However, the LCD panel having the above-described effect increases manufacturing costs of the LCD device because it needs a special LCD panel in addition to the common LCD panel.
To solve this problem, a vertical alignment LCD device has been devised. The vertical alignment LCD device will be described with reference to FIGS. 9 to 12.
FIG. 9 is a view illustrating a shape of a liquid crystal molecule when viewing a vertical alignment LCD device from the front. In a voltage non-applied state, the liquid crystal molecule is vertically aligned (see FIG. 9A). In a voltage applied state, the liquid crystal molecule is slanted upward (see FIG. 9B). An absorption axis of a polarizer is directed up and down, and the absorption axis of an analyzer is directed left and right.
FIG. 9A illustrates the vertically aligned liquid crystal molecule in the voltage non-applied state when seen from the front. In this state, birefringence of the liquid crystal molecule is not generated, and light leakage does not happen.
FIG. 9B shows the liquid crystal molecule in the voltage applied state when seen from the front. In this state, an optical axis of the liquid crystal molecule is parallel with the absorption axis of the polarizer. Similarly, birefringence is not generated, and light leakage does not happen.
FIG. 10 is a view illustrates the shape of the liquid crystal molecule when viewing the vertical alignment LCD device from the side.
As illustrated in FIG. 10A, in the voltage non-applied state, because the optical axis of the liquid crystal molecule is parallel with the absorption axis of the analyzer, light leakage does not happen.
On the other hand, as illustrated in FIG. 10B, in the voltage applied state, because the optical axis of the liquid crystal molecule is misaligned from the absorption axis of the polarizer or the analyzer, birefringence is generated, and light leakage happens.
As a result of such light leakage and because display contrast falls extremely low in left and right directions, the display is invisible when seen from the left or right slanted viewing direction. By using this phenomenon, confidentiality of the display can be controlled.
FIG. 11 is a view illustrating a basic constitution for controlling confidentiality of the display.
As illustrated in FIG. 11, one pixel includes RGB (Red, Green and Blue) subpixels, and an additional W (White) subpixel.
FIG. 12 is a view illustrating arrangement of liquid crystal molecules in the respective subpixels depicted in FIG. 11. As illustrated in FIG. 12, the liquid crystal molecules in the W subpixel are arranged in up and down directions, which is different from the arrangement of the liquid crystal molecules in the RGB subpixels.
When voltage is not applied to the W subpixel, because the W subpixel has no effect on a display, a common display appears such that the display is visible from different directions.
On the other hand, when voltage is applied to the W subpixel, the white areas are displayed in the left and right directions. As a result, the display contrast falls in the left and right directions, and the display is invisible to others who look at the display from the left/right slanted viewing direction.
FIG. 13 is a view schematically showing a basic structure of the related art vertical alignment LCD device.
As illustrating in FIG. 13, a transparent electrode which is disposed on an upper side is formed with a structure like a rib or a protrusion.
Another transparent electrode, which is disposed on a lower side, is formed with slits which have a width of about 10 microns. In the voltage non-applied state, the liquid crystal molecules are vertically aligned. In the voltage applied state, the liquid crystal molecules are aligned in a slanted direction which is determined by an influence of a slanted electric field by the structure (the protrusion or the slits), i.e., in a direction perpendicular to the extending direction of the structure (the protrusion or the slits).
However, the above related art liquid crystal display device has the following problems.
Forming the W (white) subpixels requires newly a white resin to be newly formed and a driving method is also changed.
Also, while the contrast in the left and right directions is lowered, the contrast in the up and down directions is not lowered.